Interventional cardiology, interventional angiology and other interventional techniques in cardiovascular and other vessels, ducts and channels of the human body have demonstrated marked success in recent years. Studies of interventions in the treatment of acute myocardial infarction (MI), for example, indicate the effectiveness of primary angioplasty. Implantation of coronary stents has improved the outcome of such interventional treatment. For example, these results are described in an article in the Journal of American College of Cardiology 2000, 36: 1194-1201.
Stents are being implanted in increasing numbers throughout the world to treat heart and cardiovascular disease, and are also coming into greater use outside strictly the field of cardiology. For example, other vascular interventions utilizing stents which are proving to be of equal importance to use in cardiology include stenting of the carotid, iliac, renal, and femoral arteries. Moreover, vascular intervention with stents in cerebral circulation is exhibiting quite promising results, especially in patients suffering acute stroke.
Stents are implanted in vessels, ducts or channels of the human body to act as a scaffolding to maintain the patency of the vessel, duct or channel lumen. A drawback of stenting is the body""s natural defensive reaction to the implant of a foreign object. In many patients, the reaction is characterized by a traumatic proliferation of tissue as intimal hyperplasia at the implant site, and, where the stent is implanted in a blood vessel such as a coronary artery, formation of thrombi which become attached to the stent. Each of these adverse effects contributes to restenosisxe2x80x94a re-narrowing of the vessel lumenxe2x80x94to compromise the improvements that resulted from the initial re-opening of the lumen by implanting the stent. Consequently, a great number of stent implant patients must undergo another angiogram, on average about six months after the original implant procedure, to determine the status of tissue proliferation and thrombosis in the affected lumen. If re-narrowing has occurred, one or more additional procedures are required to stem or reverse its advancement.
For virtually all stent implant patients it is desirable to examine and analyze the patency of the vessel lumen and the extent of tissue growth within the lumen of the stent, and to measure blood flow therethrough, from time to time as part of the patient""s routine post-procedure examinations. Current techniques employed to analyze patency of the lumen following a stent implant procedure are more or less invasive.
Among these techniques is vascular puncture, which, despite a relatively low complication rate, poses inherent risks as well as discomfort of the patient, such as a need for compression of the puncture site. Use of iodine containing contrast dye also prestents the possibility of negative implication such as renal failure, especially in patients with diabetes. If contrast dyes are applied to a cerebral perfusion, tissue damage may cause neurological seizures and temporary cerebral dysfunction. Therefore, it is advantageous to determine the vascular status and the functional and morphological capacity of the vascular bed by less or non-invasive methods, including methods not requiring application of iodine containing contrast dye.
Fluoroscopic techniques are an unsuitable substitute or alternative for the invasive methods because the metal stent itself causes blockage of the x-rays. Although visualization of the stent is achieved by its fluoroscopic portrayal as a shadow during the original implant procedure, the stent""s very presence defeats subsequent examination of the interior condition of the stent and the vessel lumen at the implant site by means of fluoroscopy following the implant procedure, without the use of contrast dye applied intravascularly.
Magnetic resonance imaging (MRI) can be used to visualize internal features of the body if there is no magnetic resonance distortion. MRI has an excellent capability to visualize the vascular bed, with particularly accurate imaging of the vascular structure being feasible following the application of gadolinium, a contrast dye which enhances the magnetic properties of the blood and which stays within the vascular circulation. This has special implications for the perfusion in vessels which are in a stable and resting state, especially iliac, femoral, carotid, and cerebral perfusion. On occasion of acute cerebrovascular stroke, the diagnosis of a blocked artery can be achieved quickly, within minutes, by means of an MRI technique following the intravenous injection of 30 milliliters (ml) of gadolinium.
Imaging procedures using MRI without need for contrast dye are emerging in the practice. But a current considerable factor weighing against the use of magnetic resonance imaging techniques to visualize implanted stents composed of ferromagnetic or electrically conductive materials is the inhibiting effect of such materials. These materials cause sufficient distortion of the magnetic resonance field to preclude imaging the interior of the stent. This effect is attributable to their Faradaic physical properties in relation to the electromagnetic energy applied during the MRI process.
It is a primary aim of the prestent invention to provide a stent structure and method that enables imaging and visualization of the inner lumen of an implanted stent by means of an MRI technique without need for X-ray or contrast dye application. A related aim is to enable analysis and evaluation of the degree of tissue proliferation and thrombotic attachment within the stent, and thereby, calculation of the extent of restenosis within the stent, as well as to measure the degree of blood flow, using only MRI and electromagnetic measurement of blood flow.
In German application 197 46 735.0, which was filed as international patent application PCT/DE98/03045, published Apr. 22, 1999 as WO 99/19738, Melzer et al (Melzer, or the 99/19738 publication) disclose an MRI process for represtenting and determining the position of a stent, in which the stent has at least one passive oscillating circuit with an inductor and a capacitor. According to Melzer, the resonance frequency of this circuit substantially corresponds to the resonance frequency of the injected high-frequency radiation from the magnetic resonance system, so that in a locally limited area situated inside or around the stent, a modified signal answer is generated which is represtented with spatial resolution. However, the Melzer solution lacks a suitable integration of an LC circuit within the stent.
Therefore, it is another significant aim of the prestent invention to provide a structure which enhances the properties of the stent itself to allow MRI imaging within the interior of the lumen of the implanted stent.
The prestent invention resides in a stent configuration and method of use thereof that allows imaging and visualization of the interior of the lumen of the stent after implantation in a body. Interior structures of primary interest and concern consist of body tissue build-up, thrombus formation and the characteristics of blood flow. The imaging is made feasible by a novel stent configuration which includes a tubular scaffolding structure that provides mechanical support for the vessel, duct or channel wall after the stent is deployed at a target site, and additional electrical structure which overlies the mechanically supportive tubular structure. An electrically inductive-capacitive (LC) circuit which is resonant at the magnetic resonant frequency of the MRI energy is formed by a predetermined geometric configuration of an electrically conductive layer overlying the primary mechanically supportive layer of the tubular stent structure or scaffolding of low ferromagnetic property. The two layers are separated from one another by an electrically insulative layer. This structure enables imaging and visualization of the interior of the stent by the non-invasive MRI technique.
In one of its aspects, then, the invention resides in a stent constructed and adapted to be implanted in a vessel, duct or channel of the human body as a scaffolding to maintain patency of the lumen thereof, wherein the stent comprises a mechanically supportive tubular structure composed at least primarily of metal having relatively low ferromagnetic property, and at least one electrically conductive layer overlying at least a portion of the surface of the tubular structure to enhance properties of the stent for MR imaging of the interior of the lumen of the stent when implanted in the body. An electrically insulative layer resides between the surface of the tubular structure and the electrically conductive layer. The tubular structure with overlying electrically conductive layer and electrically insulative layer sandwiched therebetween are arranged in a composite relationship to form an LC circuit at the desired frequency of magnetic resonance. The electrically conductive layer has a geometric formation arranged on the tubular scaffolding of the stent to function as an electrical inductance element and an electrical capacitance element.
In a preferred embodiment of the prestent invention, the tubular scaffolding structure is composed of niobium with a trace amount of zirconium for added strength. The thickness of this structure is preferably up to approximately 100 microns (micrometers, or xcexcm). The electrically insulative layer is an oxide of the metallic material composing the scaffolding, e.g., a layer of niobium oxide or niobium-zirconium alloy oxide, having a thickness of less than about one xcexcm, and the electrically conductive layer overlying this insulative layer is preferably composed of niobium, with a thickness of less than about 10 xcexcm. It is important to avoid electro-galvanic potentials between the scaffolding and conductive structures.
The LC circuit integrated within the stent structure according to the principles of the prestent invention further reduces the already low ferromagnetic properties of the stent and at the magnetic resonant frequency, to enhance visualization of body tissue and tissue growth within the lumen of the implanted stent during the magnetic resonance imaging. The LC circuit also enables measurement of the blood flow through the lumen of stent implanted in a blood vessel.
The LC circuit is alternatively formed as a bird cage or saddle coil pattern.